In some internal combustion engines, a lift amount of a valve is adjusted by switching between two types of cams, a high cam and a low cam, which are installed in a camshaft that operates intake and exhaust valves, in accordance with the rotational frequency of an internal combustion engine. The switching of the cams is carried out by engaging a regulating pin with a spiral groove formed in a cam to slide the cam in the axial direction of the camshaft. To operate the regulating pin, an electromagnetic actuator is used (see, e.g., Patent Literature 1).
The electromagnetic actuator described in Patent Literature 1 includes two plungers that are coupled with two regulating pins, two permanent magnets that attract the two plungers in a direction away from a cam, one coil that reduces the attraction force of the permanent magnets to allow the plungers to advance toward the cam, and two springs that cause the two plungers to advance. In an initial state, the coil is not energized and the two plungers are held by being attracted to the two permanent magnets. Since the two permanent magnets are disposed, with magnetic poles thereof being reversed to each other, the attraction force of one of the permanent magnets is reduced by energizing the coil, the plunger attracted to the permanent magnet advances with the biasing force of the spring, and the regulating pin engages with a spiral groove of the cam. After the energization to the coil is interrupted, when the regulating pin is pushed back by the cam, the plunger is attracted to the permanent magnet and is held at an initial position. Thus, the attraction force of one of the two permanent magnets is reduced by switching the energization direction of the coil, and the regulating pin on the side of the permanent magnet of which the attraction force is reduced is actuated.
In addition, the electromagnetic actuator described in Patent Literature 1 includes a function of detecting a plunger position. When the regulating pin is pushed back, the magnetic flux of the permanent magnet flowing along a magnetic path that passes through the coil is increased as the plunger approaches the permanent magnet, whereby a back electromotive force is generated between the two ends of the coil. The position of the plunger is detected by detecting the back electromotive force.